Longitudinal differential protection based on the Universal Line Model

HVDC breakers, capable of interrupting DC fault currents within only a few milliseconds are currently under development. Application of such breakers allows for selective DC fault clearing which previously has limited the practical application of large-scale DC grids. However, for reliable operation of such systems, the underlying protection algorithms must be able to distinguish between different faults so that correct DC breakers are used to clear the fault. This paper introduces a longitudinal differential protection algorithm which is derived in the frequency domain from the transmission line equations. For translation into the time domain, it relies on the techniques used by the Universal Line Model, known to accurately represent the frequency dependent behavior of both cables and overhead lines. Since the protection accounts for the distributed parameters of transmission lines, it is inherently insensitive to external faults or disturbances. This allows for implementation with fast detection times, particularly desired in multi-terminal HVDC systems.